Viruses using the enzyme reverse transcriptase for replication are retroviruses and also hepatitis B virus (HBV), which latter contains a specific DNA polymerase required for vital replication. This DNA polymerase also acts as a reverse transcriptase during the replication of HBV DNA via an RNA intermediate.
Infections by a retrovirus referred to as HIV [(Human Immuno Deficiency Virus, formerly known as Human T-cell lymphotropic Virus (HTLV-III) or Lymphadenopathy Associated Virus (LAV)] finally result in AIDS, the Acquired Immuno Deficiency Syndrome, which is characterized by a profound immunodeficiency due to low numbers of lymphocyte-T-helper cells, which are the targets for HIV (also called HTLV-III/LAV) infection. Other retroviruses affecting humans are HTLV-I and II and examples of retroviruses affecting animals are feline leukemia virus (FLV) and equine infectious anaemia virus (EIAV) and simian immunodeficiency virus (SIV).
Hepatitis B virus infections cause severe disease such as acute hepatitis, fulminant hepatitis and chronic hepatitis in a considerable number of persons. It is estimated that there are 200 million patients with chronic hepatitis B in the world. A considerable number of the chronic cases progress to liver cirrosis and liver tumours. In some cases the hepatitis infections also take a rapid and severe course as in fulminant hepatitis with about 90% mortality. At present there is no known effective treatment against hepatitis B infections.
It is also believed that multiple sclerosis, psoriasis and tropical spastic paraparesis are due to an infection with retroviruses, however, not yet identified.